The present invention relates to an optical measuring system to determine the concentration of liquid samples, especially the concentration of turbid liquid samples
In absorption spectrometry (absorption photometry) the attenuation of light upon passing through a liquid sample is measured and displayed as extinction. The attenuation of the incident light occurs by absorption, the direction of light at this point remaining unaltered.
Turbidity measurement serves to determine the scattering centers in turbid liquid samples. For this purpose one can measure the intensity of scattered light that emerges in a defined angle from the sample hit by an incident light beam. The scattering of the light can be determined either by measuring the intensity decrease of the incident light beam after passing through the turbid medium, or by measuring the intensity of the laterally deflected light. In the first case, one speaks of the method of turbidimetry, and in the second of that of nephelometry. An important application is the measurement of the concentration of liquid cell- or bacteria cultures.
With the known optical measuring systems for absorption spectroscopy and with those for turbidity measurement, it is disadvantageous that they comprise special measuring cells for the accommodation of liquid samples, which have to be arranged in an optical path between a light emitter and a light sensor for measuring, and must be produced very accurately to maintain the error in measuring as small as possible. These measuring cells are conveniently cuvettes The relatively expensive cuvettes are in principle reused. To avoid contaminations and carry-over they must be subjected to complicated cleaning procedures.
However, according to German application DE 195 35 046 C2 the measuring cell can also be a pipette point with two plane-parallel windows which is arranged in the optical path of a photometer integrated into the hand-pipette when affixed thereto. The pipette point can be made from plastic material as an one-time-use article, but this has the disadvantage that it is also relatively complicated in manufacture because of the plane-parallel windows and has a reduced accessability as a special article.
Especially the known optical measuring systems for the measurement of turbidity have the disadvantage to exhibit only a limited measuring range. Extra turbid samples can often be measured only after preceding dilution. However, the dilution of liquid samples is labour intensive and may be problematic because of possible environment-alterations of the cultures. Further, in a relatively large measuring range only a minor differentiation of small concentration differences is recorded. Indeed, this can be remedied to by using special cuvettes with different path-lenghts But this is relatively labour and apparatus expensive.
Departing therefrom the present invention has the objective to provide an optical measuring system for the determination of the concentration, especially of turbid liquid samples, simply to operate and working accurately over a large measuring range and which can be practiced with less expensive measuring cells.
The objective is solved by an optical measuring system with the features of claim 1. Advantageous embodiments of the optical measuring system are indicated in the subclaims.
The optical measuring system for the determination of the concentration especially of turbid liquids comprises:
a measuring volume for taking up the liquid samples to be measured,
a plurality of several photometric channels, each having a light source and a light sensor on different sides of the measuring volume aligned on a common optical axis, and the optical axes therefrom being disposed under different azimutal angles respective to the sample volume, and
an analysing device which evaluates the concentration of the liquid samples to be measured according to the data provided by the light sensors belonging to a plurality of different photometric channels.
Because the measuring data of a plurality of light sensors are drawn on for the determination of the concentration, inaccuratenesses (e.g. optical inhomogeneities, gauge fluctuations etc.) of a measuring cell containing the measuring volume, the walls thereof being irradiated by the photometric channels, which can especially be caused by manufacturing, are less effective in respect to the accurateness of the determined concentration. Consequently measuring cells which are manufactured with relatively little expenditure, and thus may be disposables, can be used Consequently, particularly commercially available vessels (e.g. reaction tanks, pipette points) can be used, which are widely common in laboratory use and the availability thereof is particularly high In principle, measuring cells with different shapes, e g circular or polygonal cross-section, can be used.
Particularly advantageous is the design of the measuring cell as a pipette point made from glass or plastic material. Pipette points are especially wide-spread as one-time-use articles in the plastic material make. They have at one end an aperture for the passage of liquid, and at the other end an aperture for the connection of a pipetting apparatus comprising an expulsion unit. By means of the expulsion unit, an accurately defined liquid volume can be aspirated through the point aperture into the pipette point and subsequently can be ejected again from it. The determination of the concentration in a pipette point has the particular advantage to facilitate the sample handling and to be connectable with an accurate metering of the sample liquid. In this manner one procedure can be saved, because a liquid sample conveniently is pipetted into a cuvette anyway. Also, by simply expulsing the liquid sample from the pipette point, a practically complete and contamination-free recovering of the liquid sample is possible
The analysing device can determine the concentration particularly simply based upon an averaging of data of a plurality of light sensors, e.g. based upon an arithmetrical signal averaging.
Preferably, the analysing device can determine, in a first analysing mode, concentrations based upon measuring values provided by the corresponding light sensor of the transmitted light originating from the light source belonging to the same photometric channel as the light sensor. The first analysing mode corresponds to absorption photometry, or turbidimetry respectively, and is preferably selected for an absorption-photometric measurement in the integral measuring range, and for a turbidity measurement in the range of small concentrations.
Furthermore, the analysing device can preferably determine, in a second analysing mode, concentrations based upon data which the corresponding sensor provides of scattered light that originates from at least one light source which is not belonging to the same photometric channel as the light sensor The second analysing mode corresponding to the method of nephelometry can be preferably selected if larger concentrations are to be determined by turbidimetry. In this case the range of larger concentrations can connect immediately to the range of smaller concentrations, respectively the ranges can overlap, so that added together a significantly larger measuring range can be attained by the turbidity measurement. At the same time, a good discernibility of small concentration differences is made possible over the entire measuring range.
In this way the optical measuring system enables the concentration determination of turbid liquid samples to be performed over a wide concentration range Thereby the taking into account of the data of a plurality of photometric channels at the absorption photometric measurement, and at the turbidimetric measurement in the range of smaller concentrations, and the taking into account of a plurality of measuring data of the laterally deflected light of the turbidity measurement in the range of larger concentrations, enables the use of measuring cells with inaccuracies, especially of disposables.
However, the optical measuring system can co-operate with convenient, more accurately produced measuring cells, if the measuring accuracy is to be further improved.
Further, the optical measuring system may comprise a submerged probe, which is submerged into the liquid which is to be measured, so that a liquid sample penetrates into the measuring volume which is situated between the light sources and the light sensors of the submerged probe The submerged probe can especially be tubular. The sample liquid can penetrate through an aperture on the bottom side into the inner space of the submerged probe, where the sample volume is situated. An inner space of the submerged probe comprising the measuring volume can be vented above the photometric channels, so that the sample liquid reaches the measuring volume automatically upon submersion. To protect against contamination, the measuring system of the submerged probe, respectively the photometric channels, can be surrounded by an envelope being transparent in at least the region of the optical measuring. The same can be made interchangeably, especially as a disposable (e.g. of plastic material).
According to another embodiment, in a third analysing mode, the analysing device executes the same determinations as in the first and second analysing mode, and determines the concentrations by combining the results of the two determinations. The third analysing mode is particularly suited for the determination of medium concentrations with turbidity measurements.
Preferably the optical measuring system has a switching device, enabling determination of concentrations according to one of the different analysing modes of the analysing device. This switching device can be manually-operated and adjustable by the user after visual inspection or sample measuring of the liquid sample. But it can be also a switching device which executes a switching between the different ranges automatically, according to whether smaller, larger or medium concentrations are to be determined, respectively For this purpose, in the analysing device e.g. range limits of the concentrations for the use of the different analysing modes can be stored. Then a sample measurement can be performed. If the determined concentration does not fall within the limits of an initially used analysing mode, analysing can automatically be performed with another analysing mode, until a concentration is determined which falls into the limits of the analysing mode used.
According to another embodiment, the optical measuring system can contain a controlling unit which modulates the light sources of the photometric channels, the analysing unit can detect the influence of the ambient light according to the measuring data measured by the light sensors, and eliminate the same upon the determination of the concentrations. The data measured by the light sensors can be evaluated, e.g., using the lock-in principle Another possibility is to make the modulation as large that the light source is temporarily switched out. The signals measured with the light sensors in the dark phases can then be subtracted from those measured in the illuminated phases, thereby eliminating the influence of ambient light.
According to a preferred embodiment, a controlling unit switches on the light sources of the different photometric channels consecutively, in order to avoid cross-over between the different photometric channels. Herewith it is understood with a cross-over that a light sensor of one photometric channel detects a light signal of a light source of another photometric channel, not caused by the scattering of a liquid sample. To avoid cross-over, the light sources of the different photometric channels can be modulated in a phase-shifted manner, so as to switch them on in a temporally interlocked manner. For the same reason, the light sources of the different photometric channels can be modulated with different frequencies, and an unambiguous signal assignment can be performed by frequency analysis. Further, in order to avoid a cross-over the photometric channels can pass through different cross-sectional planes of the measuring volume
Photometric channels passing through different cross-sectional planes of the measuring volume can also be present to obtain more data and e g. so further reduce the influence of inaccuracies of a measuring cell upon the measuring result.
Besides, in order to avoid a crossover and for the reduction of ambient light influences, a diaphragm element surrounding the measuring volume or the measuring cell and matching the outer contour thereof can be present, with apertures corresponding to the photometric channels.
The determination of the extinction with absorption photometry, correspondingly of the apparent extinction with turbidimetry, comprises a blank measurement (zero value measurement), that means the determination of the intensity of the transmitted light without the presence of an absorbing or scattering material (blank) in the measuring volume. Further, the determination of the extinction, or the apparent extinction correspondingly, comprises the measurement of the intensity of the emergent (attenuated) light after passing through the absorbing resp. scattering material. Analogous holds for the measurement of the intensity of scattered light with nephelometry. An additional embodiment of the optical measuring device enables to initiate a blank measurement and/or the measurement of an absorbing or scattering liquid sample by actuation of an input unit.
Preferably, the photometric channels are aligned symmetrically towards the measuring volume. Then, the optical axis of neighbouring photometric channels can include identical angles with each other. In this manner the elimination of inaccuracies of the measuring cells is favoured and the evaluation of measuring data is simplified Preferably, an odd number of photometric channels is present, as a result of which it is possible to arrange the light sources and the light sensors consecutively one after the other around the measuring volume. Even in that manner, the elimination of influences of nonuniformity of the measuring cell towards the measurement results is favoured. Preferably, three photometric channels are present which can be arranged star-like around the measuring volume with circular cross-section, to obtain the cited effects with as little expenditure as possible.
As light sources, especially LEDs, and as light sensors, silicon diodes can be used. LEDs are easily modulable. Silicon diodes can measure even high-frequent signals accurately. Further, these components are inexpensive and space-saving. Preferably the light sources and light sensors can work at 600 nm.
Particularly, the optical measuring system can be a constituent of a pipette, so that a pipette point connected to the pipette is arranged in the optical path of the photometric channels. The optical measuring system also can be, however, an apparatus or part of an apparatus which is not a pipette and exhibits an accommodation means for inserting and removing of a measuring cell. Again, the measuring cell can be a pipette point Preferably, the apparatus, correspondingly its accommodation means, are designed in a manner that a pipette point affixed to a pipette is insertible into the optical path of the photometric channels. The apparatus can comprise means for retaining a pipette with a pipette point. Particularly the apparatus can be a desktop apparatus, into which a pipette is insertible in a similar manner as into a pipette holder. Further, the apparatus can have an input unit, which enables initiation of the measurement procedures by insertion of the measuring cell, e g. with a light barrier or a push-button.
By integration into a pipette, as well as into a separate apparatus, a compact optical measuring installation is realizable which is utilizable directly at the working place. The sample does no more have to be transported to an expensive stationary optical measuring instrument.
The evaluation device and/or the switching device and/or the controlling device(s) are preferably electronic devices.